1. Field of the Invention
The present invention relates to a television camera using two image pickup devices with different sensitivity and, more particularly, is directed to a television camera suitable for imaging or image-picking up an object having a large contrast.
2. Description of the Related Arts
One of the basic functions of the television camera is to reproduce an object faithfully which has been picked up as an image. The object imaged by the television camera, however, is of various types, and has various brightness (luminances). That is, the object is composed of components with complicated brightness and components which vary in brightness. The conventional television cameras have the technique developed to faithfully reproduce an object image even when the object is composed of components with complicated brightness and different brightness.
A block diagram of a television camera according to the related art is shown in FIG. 1. The television camera shown in FIG. 1 includes an image pickup lens system 1, an image pickup device such as CCD 3, a pre-amplifier 5, a video signal processing circuit 15, a video signal output terminal 16, a lens iris control circuit 6, an iris structure 17 in the image pickup lens system 1 and a drive circuit 18 for the image pickup device 3. FIG. 2 is a photoelectric conversion characteristic diagram showing the characteristic of the signal output versus the incident light quantity of the image pickup device 3.
The operation of this television camera will be explained. The light entering from an object not shown is applied to the image pickup device 3 through the image pickup lens system 1, and after photoelectric conversion at the image pickup device 3, provides a video signal which is applied to the pre-amplifier 5.
The video signal is amplified up to a predetermined level at the pre-amplifier 5 and applied to the video signal processing circuit 15 and the lens iris control circuit 6.
The video signal applied to the video signal processing circuit 15 is subjected to signal processing such as the addition of a sync signal from a sync signal generator not shown, and is output from the video signal output terminal 16 as a composite video signal.
The lens iris control circuit 6, on the other hand, detects the average value or the peak value of the video signal input from the pre-amplifier 5, and compares the detected the average or peak value value with a reference value to extract the change in the average value or the change in the peak value, as the case may be. In accordance with the change in the average value or the peak value of the video signal thus extracted, the aperture state of the iris structure 17 in the lens system 1 is controlled in such a manner as to maintain a predetermined value of the output video signal level of the pre-amplifier 5, thereby controlling the quantity of the light entering the image pickup device 3.
Explanation will be made about the photoelectric conversion characteristic diagram showing the characteristic of the signal output versus the incident light quantity of the image pickup device shown in FIG. 2. As shown in FIG. 2, the image pickup device outputs a signal which increases substantially in proportion to the incident light quantity when the incident light quantity is below a certain level Is. When the incident light quantity is equal to or above the certain level Is, however, the signal output reaches a saturated level where the signal output does not substantially increase with the increase in the incident light quantity. This level Is at which the output signal of the image pickup device saturates will hereinafter be called as a saturation or saturated incident light quantity.
When the quantity of light entering the image pickup device from an object exceeds the saturated incident light quantity, the output signal level of the image pickup device is saturated at an image portion of the object an incident light quantity thereof exceeds the saturated incident light quantity. When this signal output is viewed on a video monitor, the image portion of the object whose incident light quantity exceeds the saturated quantity assumes a monotonically white image lacking variations and cannot be viewed as an image proportional to the incident light quantity.
The television camera, which uses an image pickup device having the characteristics described above, includes a lens iris control circuit for controlling the aperture state of the lens iris structure in such a manner that the incident light quantity entering the image pickup device from a bright object to be imaged becomes proper value. In this manner, the quantity of the light entering the image pickup device is maintained below a saturated incident light quantity level to prevent a bright portion of the object from forming a monotonically white image without any variations.
In the conventional television cameras, however, in spite of the fact that the aperture state of the iris structure in the lens system is controlled to control the incident light quantity entering the image pickup device, the incident light entering the image pickup device cannot be controlled so as to be not larger than the saturated incident light quantity Is in the case where an object has a very bright area when the object has a large contrast or the object is imaged under reverse light. As a result, the signal output is saturated, and the object as viewed on an image monitor is monotonically white without any variations. In addition, the lens iris control circuit, which is influenced by the video signal in the high brightness area, cannot properly control the incident light quantity of the object to be imaged, with the result that the object to be imaged is blackened.
In other words, when an attempt is made to obtain an image of an object of high illuminance at a proper level, an object of low illuminance becomes dark in the phenomenon of what is called the solid blackening. An attempt to produce an image of an object of low luminosity at the proper level, on the other hand, results in a high-illuminance object being whitened monotonically in a phenomenon of what is called the solid whitening.
FIGS. 3 and 4a are diagrams showing other example configuration of the television camera suggested considering the disadvantages of the prior art described above. The television camera configured this way is disclosed, for example, in JP-A-4-354277 (reference (1)).
In FIG. 3, reference numeral 1 designates a lens, 3 a CCD, 5 a pre-amplifier for amplifying the output of the CCD 3, 24 a CCD control circuit for controlling the storage time, i.e., the shutter speed of the CCD 3, 7 an analog-to-digital (hereinafter referred to as A/D) converter, 26 a high-speed image memory for writing and reading the digital video signal obtained upon the high shutter speed operation of the CCD 3 with a shorter charge storage time, and 27 a low-speed image memory for writing and reading the digital video signal obtained upon a low shutter speed operation of the CCD 3 with a longer charge storage time. Reference numeral 28 designates a high brightness segmented area detection circuit for receiving the digital video signal upon the low shutter speed operation to divided an image of the digital video signal corresponding to one screen of an object image picked-up by the CCD 3 into plural segmented areas and for detecting a high-brightness segmented area such as a back-light area among the plurality of segmented areas, 29 a signal switcher for selecting a digital video signal upon the low or high shutter speed operation, 14 a digital-to-analog (hereinafter referred to as D/A) converter, 22 an automatic gain control (AGC) amplifier, 15 a signal processing circuit for processing the video signal into a composite video signal, 16 a composite video signal output terminal, 6 an iris control circuit, and 17 an iris.
In this configuration, the CCD 3 is controlled by the CCD control circuit 24 to be alternately operated in high- and low-speed shutter modes for each field. As a result, the CCD 3 performs the high-speed operation in even-number fields to output a video signal not saturated even for the high-brightness portion of the object to the high-speed image memory 26. In odd-number fields, on the other hand, the CCD 3 performs the low-speed shutter operation, so that the video signal saturated for the high-brightness portion of the object is output to the low-speed image memory 27. The high-brightness segmented area detection circuit 28 divides an image of the digital video signal corresponding to one screen into a number N to the horizontal scanning direction and a number M to the vertical scanning direction (both N and M are integers of 2 or more). In this way, each image corresponding to one screen is composed of areas segmented into a number N.times.M of segmented areas. With each segmented area thus segmented, the average value of the digital video signal (the output of the low-speed image memory 27) upon the low-speed shutter operation is determined thereby to detect whether each segmented area is a high-brightness segmented area or not. In the case where it is decided that a segmented area is a high-brightness segmented area, the signal switcher 29 selects the output of the high-speed image memory 26 (the digital video signal for high-speed shutter operation) for the entire area of the high-brightness segmented area. In the case where decision is that a segmented area is not a high-brightness segmented area, on the other hand, the signal switcher 29 selects the output of the low-speed image memory 27 (the digital video signal for low-speed shutter operation).
FIG. 4B is a model diagram showing an image of an object corresponding to one screen 80 imaged by the CCD 3. As shown in FIG. 4B, an image of the screen 80 is segmented into nine areas 80A to 80I, for example. An image of the object, for example, is assumed to include a high-brightness area 81 and a low-brightness area 82. As a result, in this case, since the segmented area 80E is entirely covered by the high-brightness area 81 and almost of the segmented area 80E is covered by the high-brightness area 81, the segmented areas 80E and 80F, for example, are determined to be a high-brightness segmented area, so that the switcher 29 selects the digital video signal for the high-speed shutter operation supplied from the image memory 26 as the digital video signal for these segmented areas 80E and 80F. For the other segmented areas, decision is made to be not a high-brightness area and hence the digital video signal of the low-speed shutter operation supplied from the image memory 27 is selected as the digital video signal for these other segmented areas.
FIG. 4A is a circuit diagram showing the essential parts of a modification of the circuit of FIG. 3. The essential parts shown in FIG. 4 include a high-speed shutter operation control circuit 25A for controlling the control circuit 24 which in turn controls the shutter speed of the CCD 3 upon a high-speed shutter operation, and a low-speed shutter operation control circuit 25B for controlling the control circuit 24 to control the shutter speed of the CCD 3 upon a low-speed shutter operation.
The high-speed shutter operation control circuit 25A operates in a manner that an image of the output video signal corresponding to one screen from the pre-amplifier 5 is divided into three sections to the horizontal and vertical directions to obtaine nine segmented areas, for example. The average value of the digital video signal is determined for each segmented area, and the shutter speed of the CCD upon the high-speed shutter operation is controlled in such a manner that the average value of the digital video signal for a segmented area with the highest average value attains 80% of the full scale of the output voltage of the CCD, for example. The low-speed shutter control circuit 25B, on the other hand, operates in a manner that an image of the output video signal corresponding to one screen from the pre-amplifier 5 is also divided into nine segmented areas and the shutter speed of the CCD upon the low-speed shutter speed operation is controlled in a manner that the average value of the digital video signal represents 30% of the full scale of the output voltage of the CCD, for example, for the segmented area with the lowest average value of the digital video signal.
In this way, the proper exposure characteristics of the CCD are obtained even at the time of imaging of an object having a large contrast or in back light, thereby preventing the white solidification or the black solidification of the video signal.
More specifically, in the configuration of FIG. 4A, as shown in FIG. 4B, an image of the output video signal corresponding to one screen 80 of the object is segmented into nine segmented areas 80A to 80I, and it is decided whether each segmented area thus segmented is a high brightness segmented area or not on the basis of the average value of the video signal for the segmented area. The shutter speed, i.e., the charge storage time, of the CCD upon the high-speed shutter operation, is thus controlled in such a manner that the video signal for the segmented area with the highest average value assumes the proper level. Also, the shutter speed upon the low-speed shutter operation is controlled in such a way that the video signal for the segmented area with the lowest average value represents the proper level.
In FIG. 4B, the segmented areas 80E and 80F are assumed to be a high brightness segmented area, and the other segmented areas to be not high brightness segmented areas. Further the segmented area 80E has the highest average value of the video signal and the segmented area 80G has the lowest average value thereof.
The configuration of still another example of the television camera suggested taking the above-mentioned disadvantages of the prior art into consideration is shown in FIG. 5. The television camera configured this way is disclosed, for example, in JP-A-5-64070 (reference (2)).
The configuration shown in FIG. 5 includes an imaging lens 1, a iris 17, a beam splitter 2 providing a spectrometer, two CCDs 3, 4, pre-amplifiers 5a, 5b, a white level compression pre-knee circuit 30, an automatic gain control amplifier (AGC) 31, an exposure control circuit 32, an amplifier 39, delay circuits 36, 37, a low-pass filter (LPF) 33, a comparator 34, a pulse-width detection circuit 35, a multiplexer 38 and a signal processing circuit 15. The optical image of the object is divided in its light intensity or quantity by the beam splitter 2 to obtain two optical images, which are then focused on the two CCDs 3, 4. The beam splitter 2 splits the light quantity of the optical image of the object from the lens 1 into the ratio of five to one and applies the two optical images to the CCDs 3 and 4 respectively. The output video signal (the output of the amplifier 5a) of the CCD 3, to which the optical image with a larger quantity of incident light is applicated, is used for exposure control by the exposure control circuit 32 on the one hand and subjected to the knee-processing by the pre-knee circuit 30 on the other hand. Further, the output video signal of the amplifier 5a is applied to the comparator 34 through the LPF to thereby be compared with a threshold voltage V.sub.TH. When the level of the output video signal is higher than the threshold voltage V.sub.TH, a signal is applied from the pulse-width detection circuit 35 to the multiplexer 38, which in turn selects and delivers the output of the delay circuit 37 (the output video signal of the CCD 4 having a smaller light quantity) instead of the output of the delay circuit 36. In this configuration, in the case where the brightness peak value of the video signal output from the CCD 4 is 40% or more of the white level (in other words, in the case where the brightness of the bright area is at least twice that of the dark area), the fitting synthesis of the image is selected, while the knee processing is selected when the brightness peak value is less than 40% of the white level. In the fitting synthesis processing, the video signal for a relatively bright area of an image represented by the video signal obtained from the CCD 3 is replaced by the video signal for a corresponding area of the image represented by the video signal obtained from the CCD 4. In the knee processing, the output video signal of the CCD 3 is compressed in its high-brightness area.
In this way, the video signal with proper image is obtained regardless of the magnitude of the brightness difference between the bright and dark areas.